Nonresonant corrections for the optical resonance frequency measurements in the hydrogen atom

The deviation of the natural spectral line profile from the Lorentz shape for the optical resonant frequency measurements is considered. This deviation leads to an asymmetry, which is mainly due to nonresonant correction to the resonant Lorentz profile. The nonresonant corrections are studied for the different types of the atomic resonant experiments. The most accurate recent optical resonance experiments are analyzed, i.e., the two-photon 1s-2s resonance excitation of the hydrogen atom with the delayed decay in the external electric field. The description of the nonresonant correction in the latter case requires the employment of QED with different in and out Hamiltonians. The nonresonant corrections for this experiment are investigated and found to be about 10⁻⁵ Hz, while the recent experimental uncertainty is 34 Hz and in the near feature is expected to be a few hertz. The projected 1s-2s resonance excitation experiment with the three-photon ionization detection (which is now in progress) is also considered.